The present invention relates generally to a method of fabricating a chamber in a material using an isotropic etching step, and in particular to a method for forming a chamber with an aperture. The combination of the chamber and aperture may be used as a nozzle in an ink-jet print head.
Ink-jet technology is used in many applications. One of the more familiar applications of ink-jet technology is in computer-controlled printers. It is generally desirable that ink-jet printers produce high-quality documents at an acceptable rate of printing. An ink-jet pen, or print head, has an array of nozzles that print in a swath as the print head is moved relative to the paper. Print quality is at least partially determined by the number and size of the ink-jet nozzles in the print head, smaller nozzles providing superior print quality, and more nozzles providing a larger swath, resulting in greater printing speed, than fewer larger nozzles. It is desirable that the print quality does not degrade over the life of the ink-jet print head. To maintain print quality, some ink-jet printers use disposable print heads with a fixed amount of ink, designed such that the ink runs out before the nozzles degrade to an unacceptable level. Utilizing a disposable print head generates waste and increases the total cost per page of an ink-jet printer.
The nozzles are typically connected to an ink supply, or reservoir. In some instances, channels or conduits bring ink into a chamber beneath the nozzle opening, or aperture. Upon a command from the printer controller, the ink is expelled through the nozzle aperture onto a page of paper or other print media.
Various ink drivers may be used to expel the ink. For example, in some printers, an electric heating element, such as a thin-film resistor, heats the ink in the nozzle chamber to vaporize (boil) a portion of the ink, forming a bubble. The bubble causes some liquid ink within the nozzle chamber to be ejected out of the nozzle aperture. When the heating element is turned off, typically after only a few microseconds, the bubble collapses and nozzle chamber refills with ink. The collapse of the bubble can create large local pressures, up to 130 atmospheres, known as cavitation, within the chamber. The effects of the cavitation, which can include damage to the chamber and to the heating element, partially depend on the configuration of the chamber and aperture.
In other printers, a piezoelectric element is used to expel ink from the nozzle. The piezoelectric element changes dimensions in response to an applied electric field, and can create a pressure within the ink chamber to expel ink out the nozzle aperture.
The nozzle shape is important in determining the ink droplet size and velocity, the response of the ink driver, which may affect the printing speed, the durability of the ink driver, the durability of the nozzle, and other aspects of the ink-jet printer. Many different approaches have been used to fabricate ink-jet nozzles. Some approaches have used multi-step electroplating to form ink cavities and nozzles. Ink-jet nozzles have also been formed using lasers to ablate a polymer nozzle material deposited on a substrate. Other approaches rely on the anisotropic etching characteristics of single-crystal materials to form a chamber shape. For example, a {100} single crystal silicon substrate may be patterned with a masking material and etched with a solution, such as potassium hydroxide solution, to form a recess in the {100} substrate bounded by {111} side walls. The {100} substrate is then bonded to another substrate that contains the ink driver after aligning the nozzle to the ink driver.
There are at least three problems arising from the above process and similar processes. First, bonding the nozzle substrate to the ink driver substrate requires precise alignment and introduces a potential delamination problem. Second, the resultant chamber shape is limited to the anisotropic etching characteristic of the material, in the above case the {111} faces, and may not be optimum for the desired nozzle. Third, the process is restricted to single crystalline materials that exhibit anisotropic etching characteristics. These materials may not be the best choice for a nozzle material. For example, they may wear out too fast, especially when used with color inks that may contain anionic (sulfonated) dies and solvents.
Therefore, it is desirable to form nozzle apertures and nozzle chambers in a material that is compatible with color inks and other liquids. It is further desirable that the nozzle chamber is suitably shaped for use in an ink-jet print head or other jet device, and that the shape of the resulting nozzle chamber may be varied according to process controls to optimize nozzle performance.
The present invention provides a method for forming a chamber in a material. The chamber may be configured to define a nozzle structure. The chamber is formed by first creating a surface feature, such as a pit or trench, on the surface of the material. A layer of resist is applied to the sidewall of the surface feature and the material is isotropically etched such that the etch works back up the inside of the resist on the surface feature sidewall to form a chamber with a re-entrant angle between the surface feature sidewall and the top of the chamber wall. This results in a chamber that is wider than the opening between the sidewalls of the surface feature. An anisotropic etch step may be performed before or after an isotropic etch step to control the final shape of the chamber.
In one embodiment, ink-jet nozzles are fabricated in a layer of silicon oxide on a silicon wafer substrate. An etch-stop layer between the silicon oxide layer and the silicon substrate forms a planar back wall of the chamber. Removing the etch-stop layer after the chamber has been formed exposes an ink driver, such as a thin film resistive heater.
In another embodiment, conduits are formed in a material by forming a trench on the surface of the material. The sidewalls of the trench are covered with resist material and a conduit is etched in the material using an isotropic etch that etches back up the inside of the sidewall resist.
These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and the attached figures.